I. Field of the Invention
This invention relates generally to communication systems. More specifically, the invention relates to multiple access communication systems.
II. Description of the Related Art
The use of wireless communication systems for the transmission of digital data is becoming more and more pervasive. In a wireless system, the most precious resource in terms of cost and availability is typically the wireless link itself. Therefore, one major design goal in designing a communication system comprising a wireless link is to efficiently use the available capacity of the wireless link. In addition, it is also desirable to reduce the delay associated with use of the link.
In a system in which multiple units compete for finite system resources, a means must be developed to regulate access to such resources. In a digital data system, remote units tend to generate bursty data. The bursty data is characterized in that it has a high peak-to-average traffic ratio, meaning that blocks of data are transferred during short periods of time interposed between significantly longer periods of idleness. Dedication of an individual communication channel to each active unit does not result in efficient use of system capacity in a system in which units generate bursty data because, during those times when the remote unit is not utilizing the system, the allocated resources remain idle. The use of dedicated channels also may impose a hard limit on the number of remote units which may simultaneously use the system, regardless of the usage patterns of the remote units. In addition, the use of dedicated channels may cause unacceptable delay if the slice of resources allocated to each remote unit is so small that data transfer rates are greatly compromised.
The characteristics of the inbound and outbound traffic tend to differ significantly in a digital data system. For example, in a system which provides wireless Internet services, a typical outbound transmission from a remote unit is relatively short, such as a request for a web page. However, a typical inbound data transfer to a remote unit tends to be rather large. For example, in response to a request for a web page, the system may transfer a significant amount of data. Because the characteristics of the inbound and outbound links are very different, system efficiency may be increased by developing two distinct protocols for the inbound and outbound links.
A random access ALOHA protocol was developed for use in the outbound link from a remote unit in a digital data system. The basic idea behind ALOHA is quite simple: the remote units transmit whenever they have data to send. If the remote units are using a communication resource which can only be accessed by one remote unit at a time, the information from each remote unit is destroyed if two units transmit at the same time causing a collision. In a system where the remote unit can monitor the random access transmissions, the remote unit may monitor the transmissions in order to determine whether its transmission is the victim of a collision. In a system in which the remote unit does not or cannot monitor the random access transmissions, the remote unit may detect a collision based upon the expiration of a timer without receipt of an acknowledgment message received from a hub station in response to a transmission. According to standard ALOHA operation, whenever a collision occurs, the remote unit waits a random amount of time and retransmits the data. The duration of the wait is random so that the colliding remote units do not generate collisions in lockstep over and over again.
FIG. 1 is a timing diagram showing the operation of a pure ALOHA random multiple access system. In FIG. 1, five remote units designated A, B, C, D and E are transmitting packets of data within a common communication channel. Whenever two remote units transmit at the same time, a collision occurs and both transmissions are lost. In a pure ALOHA system, if the first bit of a new transmission overlaps just the last bit of a transmission already in progress, both transmissions are totally destroyed and both have to be retransmitted at some other time. For example, in the frequency modulated (FM) channel shown in FIG. 1 where no two packets may contemporaneously be transmitted, a packet 12 transmitted by the remote unit B collides with a packet 10 transmitted by the remote unit A and a packet 14 transmitted by the remote unit C. The remote unit A must retransmit the information in the packet 10, the remote unit B must retransmit the information in the packet 12 and the remote unit C must retransmit the information in the packet 14. FIG. 1 shows the remote unit C retransmitting the packet 14 as a packet 14R.
In a pure ALOHA system, if the average packet transfer rate is low, most packets are transferred without a collision. As the average packet transfer rate begins to increase, the number of collisions increases and, hence, the number of retransmissions also increases. As the system loading increases linearly, the probability of retransmissions and multiple retransmissions increases exponentially. At some point as system loading increases, the probability of successful transmission falls below a reasonable number and the system becomes practically inoperable. In a pure ALOHA system, the best channel utilization which can be achieved is approximately 18%, the so-called maximum channel utilization. Below 18%, the system is underutilized. Above 18%, the number of collisions increases such that the throughput of the system begins to fall. Operating above maximum channel utilization is referred to as over channel utilization. Under conditions of over channel utilization, the average delay of the system increases rapidly as the throughput of the system falls and the stability of the system is endangered.
The introduction of a geosynchronous satellite link within a digital communication system complicates the multiple access dilemma. The use of a geosynchronous satellite typically introduces a 270 millisecond (msec) delay between transmission of a signal from a remote unit and reception of that same signal at a hub station. For this reason, scheduled access schemes which require the remote unit to request system resources before beginning each transmission introduce about half a second of delay into each transmission. The delay associated with scheduled transmissions may be readily apparent to the frustrated system user.
If an ALOHA system is implemented in a satellite system in which the remote units can't or don't monitor the random access channel, in the event of a collision, the remote unit does not know of the collision for at least 540 msec. In addition to the notification delay, the remote unit typically must wait some random amount of time before retransmitting the data (to avoid lockstep retransmissions). The retransmitted signal is once again subjected to the 270 msec time delay. The cumulative delay of such a transmission can easily exceed one second. In a fully loaded system, the delay can be significantly longer due to the increased probability of repeated collisions. Although these delays are not incurred with each transmission, they can be frustrating to the user when incurred.
Therefore, there is a need for a multiple access system which provides advantageous use of system resources as well as tolerable delay.